Return a non-zero exit code in the event of a fatal-error SSH

Return a non-zero exit code in the event of a fatal-error SSH
session termination. `Close window only on clean exit' was not
working properly on Unix in the absence of this:
notify_remote_exit() was being called and ssh_return_exitcode was
returning zero, causing gtk_main_quit() to be called, _before_
connection_fatal() happened.

[originally from svn r6801]